Blasthole drills are large machines used to drill holes for explosives in mining operations. A conventional blasthole drill comprises a frame supported by crawlers for movement over the ground, and a mast supported by the frame for movement between a substantially vertical position and a number of angled or non-vertical positions. The mast defines a drill hole axis. A rotary head moves relative to the mast along the drill hole axis. The rotary head engages the upper end of a drill pipe for rotating the drill pipe and driving the drill pipe into the ground. When drilling a blasthole that is deeper than the height of the mast, more than one section of drill pipe must be used. After the first section of drill pipe is driven into the ground, the rotary head moves back to the top of the mast and another section of drill pipe is connected to the top of the first section. The rotary pipe then drives the second section into the ground. It is not unusual to use four sections of drill pipe. Such a deep blasthole is referred to as a "multi-pass" blasthole.
After drilling a multi-pass blasthole, it can be difficult to break the joint between two pipe sections. A blasthole drill typically includes an automatic breakout wrench for breaking a joint if the rotary head cannot do so. An automatic breakout wrench is disclosed in U.S. Pat. No. 4,128,135. The automatic breakout wrench turns the upper pipe section while the lower pipe section is held by deck wrenches.
A conventional wrench includes a swing arm pivotable relative to the mast between extended and retracted positions. A wrench member pivots relative to the swing arm about the drill hole axis when the swing arm is in the extended position. The wrench member carries dies for gripping the pipe section. Movement of the wrench member relative to the swing arm is guided by two pins which extend from the wrench member and which move in arcuate slots in the swing arm. A clamping jaw pivots relative to the wrench member between a clamping position and a non-clamping position. The jaw carries a die for gripping the pipe section. When the swing arm is in the extended position, movement of the jaw to the clamping position causes the pipe section to be gripped by the dies on the jaw and on the wrench member. Thereafter, pivotal movement of the wrench member relative to the swing arm (the clamping jaw moves with the wrench member) turns the pipe section to break the joint. Pivotal movement of the wrench member is caused by a hydraulic breakout cylinder connected between the swing arm and the wrench member.